The metallochaperone proteins Atx1 and CCS (copper chaperone for copper, zinc superoxide dismutase, SOD1) are soluble metal receptor proteins that function to guide and protect the metal ion while facilitating appropriate partnerships within the cell. The Atx1-like proteins ensure the facile delivery of a Cu(l) cofactor to intracellular targets in the secretory pathway while CCS has a more complex structure and function. New results suggest that this metallochaperone requires oxidants such as oxygen to complete formation of the mature and active state of SOD1. Elucidating the mechanisms of these processes will provide keys to understanding the cell biology of copper in pathological conditions, such as Wilson and Menkes disease and familial amyotrophic lateral sclerosis (fALS). Kinetic, thermodynamic and structure-function studies of the metallochaperones and their physiological targets will test the hypothesis that these proteins function by lowering the activation barrier for Cu-transfer to partner proteins but maintain high barriers for transfer to other sites. While the multidomain copper chaperone CCS neither detoxifies copper or reactive oxygen species (ROS), new results suggest that it plays a role in posttranslational regulation of oxidative stress responses: as oxidative stress increases, CCS facilitates the correct disulfide bond formation in its target, apoSODL These mechanistic, physiological and structural studies will provide the bais for a more complete understanding of metal trafficking and homeostasis in disease. The newly developed tools and reagents will be used to address roles of copper proteins in neurodegenerative diseases, as well as the emerging connections between copper cell biology and oxygen physiology. For instance these studies will test an emerging model for the gain of function mutations in SOD1 that cause fALS: the immature disulfide reduced forms of the disease causing proteins are completely unfolded at physiological temperature and readily become insoluble aggregates upon formation of inappropriate disulfide crosslinks.